Prepare chat.api 0.1.0 for CRAN

[TroyHernandez]

Prepare the first CRAN release as 0.1.0, with published Matrix dependency
floors, runnable examples, and portable package tests. Consolidate the
prerelease NEWS entries under 0.1.0 and record submission checks in
cran-comments.md.

CI installs every suggested adapter package from CRAN, builds the Matrix
packages from published sources to avoid binary lag, verifies their versions,
and fails on check warnings. Linux installs the libcurl headers needed by its
source dependencies. Tests use temporary user directories and handle Windows
permissions and normalized upload paths on macOS.

Validation:

• Linux R CMD check --as-cran --run-donttest: 0 errors, 0 warnings, 1
  New submission NOTE; 1,414 assertions passed with all suggested packages.
• Without optional adapter packages: 1,039 assertions passed.
• Previous candidate on Windows R-devel: 0 errors, 0 warnings, 1 New
  submission NOTE; all 1,413 Windows assertions passed.
• tinyrox regeneration leaves Rd files and NAMESPACE unchanged.
• Hosted Linux and macOS CI
  both pass with Status: OK, using mx.api 0.3.1, mx.client 0.2.1, and
  mx.crypto 0.2.2.

CRAN submission remains pending a clean Windows R-release recheck. Its last
run passed 1,403 assertions but warned about mx.crypto::mxc_signing_key_public;
the builder's older mx.crypto skipped the 10 newer crypto assertions.
The required export is present in CRAN source mx.crypto 0.2.2. The final
archive includes the normalized upload-path test fix and needs a Windows
R-release check before Troy submits it to CRAN.